Integrated opto-electronic oscillators having optical resonators

ABSTRACT

Systems and techniques of incorporating an optical resonator in an optical part of a feedback loop in opto-electronic oscillators. This optical resonator provides a sufficiently long energy storage time and hence to produce an oscillation of a narrow linewidth and low phase noise. Certain mode matching conditions are required. For example, the mode spacing of the optical resonator is equal to one mode spacing, or a multiplicity of the mode spacing, of an opto-electronic feedback loop that receives a modulated optical signal and to produce an electrical oscillating signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S.application Ser. No. 09/491,988, filed Jan. 26, 2000 now U.S. Pat. No.6,567,436, which claims the benefits of U.S. provisional applicationNos. 60/117,568 filed on Jan. 27, 1999, 60/117,452 filed on Jan. 26,1999, and 60/117,451 filed on Jan. 26, 1999.

ORIGIN OF THE DISCLOSURE

The systems and techniques described herein were made in the performanceof work under a NASA contract, and are subject to the provisions ofPublic Law 96-517 (35 USC 202) in which the Contractor has elected toretain title.

BACKGROUND

This application relates to methods and devices for generation ofoscillating signals, and more specifically, to generation of oscillatingsignals by using opto-electronic oscillators.

Oscillating signals can be generated by using various types ofoscillators having energy storage elements. The quality factor Q, or theenergy storage time, of an energy storage element can determine thespectral linewidth of the respective oscillating signal. Increasing thequality factor Q or the energy storage time can reduce the spectrallinewidth of the oscillating signal and hence improve the signal'sspectral purity.

Spectrally pure radio frequency (RF) oscillators can be used forgenerating, tracking, cleaning, amplifying, and distributing RFcarriers. Such RF carriers can have important applications incommunication, broadcasting, and receiving systems in the radiofrequency spectral range. In particular, voltage-controlled RFoscillators with phase-locked loops can be used for, among others, clockrecovery, carrier recovery, signal modulation and demodulation, andfrequency synthesizing.

RF oscillators can be constructed by using both electronic and opticalcomponents to form opto-electronic oscillators (“OEOs”). See, e.g., U.S.Pat. Nos. 5,723,856 to Yao and Maleki and 5,777,778 to Yao. Such an OEOincludes an electrically controllable optical modulator and at least oneactive opto-electronic feedback loop that comprises an optical part andan electrical part interconnected by a photodetector. Theopto-electronic feedback loop receives the modulated optical output fromthe modulator and converted it into an electrical signal to control themodulator. The loop produces a desired delay and feeds the electricalsignal in phase to the modulator to generate and sustain both opticalmodulation and electrical oscillation in radio frequency spectrum whenthe total loop gain of the active opto-electronic loop and any otheradditional feedback loops exceeds the total loss.

OEOs use optical modulation to produce oscillations in frequencyspectral ranges that are outside the optical spectrum, such as in RF andmicrowave frequencies. The generated oscillating signals are tunable infrequencies and can have narrow spectral linewidths and low phase noisein comparison with the signals produced by other RF and microwavesoscillators. Notably, the OEOs are optical and electronic hybrid devicesand thus can be used in optical communication devices and systems.

A variety of OEOs can be constructed based on the above principles toachieve certain operating characteristics and advantages. For example,another type of OEOs is coupled opto-electronic oscillators (“COECs”)described in U.S. Pat. No. 5,929,430 to Yao and Maleki. Such a COEOdirectly couples a laser oscillation in an optical feedback loop to anelectrical oscillation in an opto-electronic feedback loop.Opto-electronic oscillators can also be implemented by having at leastone active opto-electronic feedback loop that generates an electricalmodulation signal based on the stimulated Brillouin scattering. U.S.Pat. No. 5,917,179 to Yao. Such a Brillouin OEO includes a Brillouinoptical medium in the feedback loop and uses the natural narrowlinewidth of the Brillouin scattering to select a single oscillatingmode.

SUMMARY

The present disclosure includes opto-electronic oscillators thatimplement at least one high-Q optical resonator in an electricallycontrollable feedback loop. An electro-optical modulator is provided tomodulate an optical signal in response to at least one electricalcontrol signal. At least one opto-electronic feedback loop, having anoptical part and an electrical part, is coupled to the electro-opticalmodulator to produce the electrical control signal as a positivefeedback. The electrical part of the feedback loop converts a portion ofthe modulated optical signal that is coupled to the optical part of thefeedback loop into an electrical signal and feeds at least a portion ofit as the electrical control signal to the electro-optical modulator.

The high-Q optical resonator may be disposed in the optical part of theopto-electronic feedback loop or in another optical feedback loopcoupled to the opto-electronic feedback loop, to provide a sufficientlylong energy storage time and hence to produce an oscillation of a narrowlinewidth and low phase noise. The mode spacing of the optical resonatoris equal to one mode spacing, or a multiplicity of the mode spacing, ofthe opto-electronic feedback loop. In addition, the oscillatingfrequency of the OEO is equal to one mode spacing or a multiple of themode spacing of the optical resonator.

The optical resonator may be implemented in a number of configurations,including, e.g., a Fabry-Perot resonator, a fiber ring resonator, and amicrosphere resonator operating in whispering-gallery modes. These andother optical resonator configurations can reduce the physical size ofthe OECs and allow integration of an OEO with other photonic devices andcomponents in a compact package such as a single semiconductor chip.

These and other aspects and associated advantages will become moreapparent in light of the detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment of a single-loop opto-electronic oscillatorhaving an optical resonator according to the invention.

FIGS. 2A, 2B, 2C, and 2D illustrate mode matching conditions for theoscillator shown in FIG. 1.

FIGS. 3A and 3B show implementations of the active control of therelative frequency between the laser and the resonator in the oscillatorshown in FIG. 1.

FIGS. 4A and 4B illustrate the operation of the frequency controlcircuit based on the error signal produced from dithering the laser orthe resonator.

FIG. 5 shows an implementation of the frequency control circuit thatreceives its input from an optical signal reflected by an opticalresonator in an opto-electronic oscillator.

FIGS. 6, 7, 8A, and 8B show examples of the optical resonator that canbe used in an opto-electronic oscillator according to the invention.

FIG. 9 illustrates an embodiment of an integrated opto-electronicoscillator with all its components fabricated on a semiconductorsubstrate according to the invention.

FIGS. 10A and 10B show two examples of dual-loop opto-electronicoscillators having an optical resonator according to the invention.

FIG. 11 shows a Brillouin opto-electronic oscillator implementing anoptical resonator and a frequency control circuit according to oneembodiment of the invention.

FIG. 12 shows the mode matching conditions of the Brillouinopto-electronic oscillator shown in FIG. 11.

FIG. 13 shows a Brillouin opto-electronic oscillator that uses a singlelaser to produce both the pump laser and the signal laser.

FIG. 14 shows an exemplary dual-loop Brillouin opto-electronicoscillator having a Brillouin opto-electronic loop and a non-Brillouinopto-electronic loop.

FIG. 15A shows one embodiment of a coupled opto-electronic oscillatorhaving an optical feedback loop and an opto-electronic loop.

FIG. 15B illustrate the spectral relationships of the resonatortransmission, the laser modes, the opto-electronic oscillation modes,and the RF filter's center frequency and bandwidth in the device in FIG.15A.

FIG. 15C shows a coupled opto-electronic oscillator having two opticalresonators based on the system in FIG. 15A.

FIG. 16 shows an embodiment of an integrated coupled opto-electronicoscillator based on a micro cavity in whispering gallery modes.

DETAILED DESCRIPTION

An optical resonator of a high Q factor, when incorporated into anoptical section of an opto-electronic oscillator as an energy storageelement, can produce a number of effects in the oscillator. First, whenintended as an energy storage element, the optical resonator caneffectuate an increase in the energy storage time of the oscillator andhence reduce the spectral linewidth and the phase noise of theopto-electronic oscillation.

Second, the optical resonator generates its own resonator modes in theoscillator, in addition to the modes caused by other feedback loops.This requires certain mode matching between the resonator modes andother modes in the oscillator so as to properly operate the overallopto-electronic oscillator.

Third, when the optical resonator is configured to have a mode spacinggreater than another mode spacing in the oscillator, the mode matchingconditions force the mode spacing of the opto-electronic oscillation tobe the mode spacing of the resonator. Hence, the cavity length of theresonator can be made so small that the mode spacing of the oscillationis sufficiently large to allow an easy selection of a single mode foroscillation by electrical filtering.

The following illustrates examples of opto-electronic oscillators ofdifferent configurations that implement an optical resonator and theirrespective mode matching conditions.

FIG. 1 shows one embodiment 100 of a single-loop opto-electronicoscillator with an optical resonator 121. This and other devices basedon a single opto-electronic loop, without the optical resonator, aredescribed in U.S. Pat. No. 5,723,856 to Yao, which is incorporated byreference in its entirety. The OEO includes a light source 101, anelectro-optic (“EO”) modulator 110, and an opto-electronic feedback loop120 coupled to the EO modulator 110. A light beam 102 from the lightsource 101 is coupled into the EO modulator 110 and is modulated inresponse to a feedback signal from the feedback loop 120 that is appliedto a driving port 115.

In the EO modulator 110, an input optical coupler 111 splits the beam102 into two channels 112 and 114 which undergo different phase orpolarization modulation. A bias port 113 is used to apply an electricalbias to offset the modulation via an external source. The lightmodulation is controlled by both the electrical voltage bias at the biasport 113 and the feedback voltage at the driving port 115. The lattercauses the light modulation to oscillate at a desired frequency such asa RF frequency. An output optical coupler 116 recombines the light fromthe modulated two channels 112 and 114 to produce a RF modulated lightsignal through coherent interference. The output coupler 116 sends oneportion of the coupled output of the modulator 110 as an optical output117 and another portion as a feedback 118 into the opto-electronicfeedback loop 120. The opto-electronic feedback loop 120 and the EOmodulator 110 forms a closed loop to support the opto-electronicoscillation.

The opto-electronic feedback loop 120 generally includes an optical part122 and an electrical part 124 that is coupled to the optical part 122.A photodetector 127 interconnects the optical part 122 and theelectrical part 124 by converting the optical signal into an electricalsignal. An optical coupler 125 may be used to couple an external opticalsignal 126 into the output of the optical part 122 so that the total sumof the optical signal from the optical part 122 and the external signal126 is measured by the photodetector 127.

The optical part 122 includes the optical resonator 121 and an opticalwaveguiding element 123 such as optical fiber. The optical waveguidingelement 123 couples the output 118 to the resonator 121 and guides theoutput of the resonator 121 to the photodetector 127. The opticalresonator 121 is configured to have a high Q factor to cause the longestdelay in the OEO 100. The oscillator 100 is configured so that the modesof the resonator 121, the oscillation frequency of the oscillator 100,and the modes of the feedback loop 120 must have desired mode matchingrelationships with one another in order to sustain the opto-electronicoscillation when the loop gain in the closed loop formed by the loop 120and the EO modulator 110 is greater than unity.

The length of the optical waveguiding element 123 primary be configuredto cause a desired delay in addition to the delay caused by the opticalresonator 121. The delay in the optical part 122 may be adjusted by,e.g., changing the cavity length of the resonator 121. In addition, thelength of the optical waveguiding element 123 can be changed to alterthe delay in the optical part 122. A fiber stretcher may be used toachieve this.

The electrical part 124 of the opto-electronic feedback loop 120includes an electrical amplifier 130 and an electrical signal band-passfilter to select a desired spectral component in the part 124 as thefeedback driving signal to the port 115. In addition, an electricalsignal coupler 140 may be provided in the part 124 to inject an externalelectrical signal 141 and to produce an electrical output 142. Moreover,a variable electrical delay 160 may be used in the part 124 to changesignal delay in the part 124 and hence the total delay in the feedbackloop 120.

The feedback from the opto-electronic feedback loop 120 to the OEmodulator 110 is positive in order to produce an oscillation. This canbe achieved by controlling the total delay or phase shift of thefeedback loop 120. The total gain in the closed loop formed by thefeedback loop 120 and the OE modulator 110 is greater than unity, i.e.,the gain exceeding losses for the circulating waves in the loop. Theloop gain can be controlled and maintained by using the electricalamplifier 130, the injected signal 126 or 141, or a combination of theabove. Notably, both electrical signal and the optical signal in theoscillator 100 are in the feedback loop 120 and hence are inherentlyinterconnected. If either signal changes, the other also changes.

In general, many modes can oscillate simultaneously in the single-loopOEO 100. The filter 150 such as a RF filter is used to achieve a singlemode oscillation by suppressing other oscillation modes. The filter 150can also be used for coarse tuning the oscillation frequency. Frequencytuning can also be achieved by biasing at the bias port 113 and thevoltage input at the RF driving port 112. The delay in the optical part122 can be used to control and finely tune the oscillation frequency.The phase noise in this single-loop OEO can be significantly reduced asthe total delay in the feedback loop 120 increases.

The cavity length of the optical resonator 121 is set to have a freespectral range, i.e. the spacing between two adjacent cavity modes, thatis greater than the mode spacing of the modes of the opto-electronicloop formed by the loop 120 and the OE modulator 110 including thecontribution from the resonator 121. Hence, the mode spacing of the OEO100 is primarily determined by the mode spacing of the resonator 121.The cavity length of the resonator 121 can be set sufficiently small sothat the resonator mode spacing is generally sufficiently large for thebandpass filter 150 to filter out unwanted modes and makes theoscillator 100 operate in single mode. The large mode spacing providedby the optical resonator 121 is also advantageous for radar and otherapplications because the unwanted residual side modes, if still exist,can be filtered out externally.

Certain mode matching conditions for the OEO 100 must be satisfied inorder to sustain an oscillation with a loop gain greater than thelosses. A mode that fails to meet the mode matching conditions issubject to significant loss caused by the spectral profiles of thepermissible natural opto-electronic oscillation modes and thepermissible resonator modes in additional to other losses in the closedopto-electronic loop formed by the feedback loop 120 and the OEmodulator 110.

The mode matching conditions include: (1) the laser center frequencyν_(laser) of the input beam 102 from the laser 101 are within one of thetransmission peaks of the optical resonator 121 so that enough light canreach the photodetector to assure the open loop gain of theopto-electronic loop greater than unity, i.e.,ν_(laser) =M·FSR _(r),where M is a positive integer and FSR_(r) is the free spectral range ofthe optical resonator 121; (2) the free spectral range, FSR_(r), of theoptical resonator is equal to one or a multiplicity of the mode spacing,Δν_(OE Loop), of the natural modes in the opto-electronic loop, i.e.,FSR _(r) =N·Δν _(OE Loop,)where N is a positive integer (1, 2, 3, . . . ); and (3) thefrequency·Δν_(OEO) of the opto-electronic oscillation of the OEO 100equals to the multiples of the free spectral range FSR_(r) of theresonator 121:ν_(OEO) =K·FSR _(r),where K is also a positive integer (1, 2, 3, . . . ).

FIGS. 2A, 2B, 2C, and 2D illustrate the above mode matching conditions.FIG. 2A shows the permissible resonator modes of the optical resonator121 in the optical frequency range. In the same frequency scale as inFIG. 2A, FIG. 2B shows the position of the laser frequency ν_(laser) ofthe input beam 102 for the EO modulator 110 to be resonant or overlapwith one of the resonator modes in FIG. 2A under the mode matchingcondition (1). FIG. 2C shows the natural modes of the opto-electronicloop in a frequency range of the opto-electronic oscillation (e.g., theRF frequency range) of the OEO 100 under the mode matching condition (2)where N=2. FIG. 2D shows the opto-electronic oscillation frequencyν_(OEO) is given by 2FSR_(r)(K=2) under the mode matching condition (3).

The EO modulator 110 modulates the input beam 102 to produce one or moremodulation side bands. FIG. 2B shows the first two orders of sidebands.Because of the mode matching condition (3), each modulation sideband isin resonance with a resonator mode. Hence, the +1 sideband overlaps witha resonator mode 214 and the −1 sideband overlaps with resonator mode212 when the laser center frequency overlaps with the resonator mode210. This automatically assures that the optical sidebands generated bythe modulator pass through the resonator with minimum loss.

When the above three mode matching conditions are satisfied, the modespacing of the OEO 100 becomes the free spectral range FSR_(r) of theresonator 121. For single mode selection, the bandpass filter 150 with abandwidth less than the free spectral range FSR_(r) of the resonator 121can be used, as shown in FIG. 2C. In certain implementations, FSR_(r) ofthe resonator 121 can be configured to be much greater than the modespacing·Δν_(OE Loop) of the opto-electronic oscillation, e.g., about 100times of that of a fiber delay line based OEO (on the order 10 MHz orlarger). Therefore, unlike in a single-loop OEO without the opticalresonator 121, the bandwidth of the RF filter 150 is not required to benarrow, e.g., bandwidth on the order of hundreds of KHz at a carrierfrequency on the order of 10 GHz which is difficult to achieve. Rather,the bandwidth of the filter 150 can be on the order of 10 MHz or greaterand such RF filters are available. Thus, the implementation of theoptical resonator 121 in the single-loop OEO 100 not only increases thetotal delay in the loop 120 to reduce the phase noise but also providesa frequency selecting mechanism to ensure a single mode operation.

In practical applications, fluctuations of the environmental conditionsand aging of the device components, such as variations in temperature,stress, or other type disturbances, can cause changes in both the laserfrequency ν_(laser) from the laser 101 and the transmission peakfrequencies of the resonator 121. Therefore, the relative value of thelaser frequency ν_(laser) and a respective resonant transmission peak ofthe resonator 121 can change over time in absence of a controlmechanism. Such a change, when exceeding a range, can destroy the modematching condition (1) and hence cause a malfunction of the OEO 100 ofFIG. 1.

Therefore, it is desirable to control the difference between the laserfrequency ν_(laser) and a respective transmission peak of the resonator121 to maintain the mode matching condition (1). This can be achieved byeither actively locking the frequency ν_(laser) of the laser 101 to therespective transmission peak of the resonator 121, or alternatively,actively locking the resonator 121 to the laser 101. The choice of thesetwo frequency locking techniques depend on which one is moreenvironmentally stable in a specific application. In both active lockingtechniques, a monitoring mechanism is used to monitor the differencebetween the laser frequency ν_(laser) and the respective transmissionpeak of the resonator 121 to generate an error signal. Then, in responseto this error signal, a frequency correcting mechanism is used to reducethe frequency difference to a value with a tolerable difference range.

FIGS. 3A and 3B show two implementations of the active control of therelative frequency between the laser 101 and the resonator 121. Bothimplementations use a frequency control circuit 310 which detects thefrequency difference and applies a control signal either to the laser101 or to the cavity of the resonator 121. The input of the circuit 310is coupled to receive an electrical signal converted from the opticaloutput of the optical resonator 121. A designated photodetector 330 maybe used to produce the input to the circuit 310. Alternatively, thisinput can be obtained from the output of the photodetector 127 thatinterconnects the two parts 120 and 124 of the feedback loop 120 of theOEO 100 in FIG. 1. One embodiment of the frequency control circuit 310includes a signal phase detector 314, a low-pass filter 316, a ditheringsignal generator 318, and a signal adder 320. A signal amplifier 312 maybe used to amplify the input signal to the phase detector 314. Thedither 318 produces a sinusoidal dither of a frequency f_(d) and iscoupled to provide the same dither signal to both the adder 320 and thephase detector 314. In operation, the phase detector 314 compares thephase of the dither signal to that of the output from the resonator 121to produce a first error signal. After being filtered by the low-passfilter 316, the first error signal and the dither signal are added toform a second error signal that is fed to either the laser 101 as shownin FIG. 3A or the resonator 121 as shown in FIG. 3B to reduce thefrequency difference.

FIGS. 4A and 4B illustrate the operation of the frequency controlcircuit 310 based on the error signal produced from dithering the laser101 or the resonator 121. FIG. 4A shows the transmission spectral peakof the resonator 121 in a relationship with the laser frequency. Whenthe laser center frequency ν_(laser) is aligned with the desiredtransmission peak of the resonator 121, the detected signal is zero atf_(d) and maximum at 2f_(d). If the laser frequency ν_(laser) is on theleft side of the transmission peak (ν<ν_(o)), the detected error signalat f_(d) is non-zero and is in phase with the applied dithering controlsignal. On the other hand, when the laser frequency is at right of thetransmission peak (ν>ν_(o)), the detected error signal at f_(d) willhave an opposite phase with that of the applied dithering controlsignal. By using a phase sensitive detection scheme by the operations ofthe phase detector 314 and the adder 320 to produce a phase sensitiveerror signal, the relative frequency of the laser 101 and the resonator121 can be actively locked within a desired range to maintain the modematching condition (1).

Other active frequency locking methods can also be used. For example,one alternative technique is described by R. Drever et al. in “Laserphase and frequency stabilization using an optical cavity,” AppliedPhysics B, Vol. 31, pp. 97-105, (1983). Another alternative technique isdescribed by and Hansch and Couillaud in “Laser frequency stabilizationby polarization spectroscopy of a reference cavity,” OpticsCommunications, Vol. 35, pp. 441-444 (1980).

In an alternative implementation, the input to the frequency controlcircuit 310 may be converted from an optical signal reflected from theresonator 121. FIG. 5 shows this configuration. An optical circulator ora beam splitter 510 can be disposed between the EO modulator 110 and theresonator 121 to direct the reflected optical signal to thephotodetector 330.

The optical resonator 121 can be implemented in different configurationsto operate as the optical energy storage component. For example, fiberFabry-Perot resonators, fiber ring resonators, optical microsphereresonators, and other micro-resonators can be used to construct an OEO.The use of optical resonators can significantly reduce the size of theOEO due to their high Q factors. In particular, the optical microsphereresonator and other types of micro resonators can be used to integratean OEO on a chip.

A compact and light weight Fabry-Perot resonator can be constructed byforming highly reflective coatings 601 and 602 on two ends of a segmentof optical fiber 603 to form a fiber Fabry-Perot resonator. FIG. 6 showssuch a fiber Fabry-Perot resonator. An alternative way to make a fiberFabry-Perot resonator is to form fiber Bragg gratings at or near bothends to replace the reflective coatings 601 and 602. Light coupling intothe resonator bounces back and forth inside the resonator before exitingso that the effective energy storage time dramatically increases. For aRF signal on an optical carrier, the effective energy storage timeτ_(eff) of the fiber Fabry-Perot resonator isτ_(eff)=τ_(d)·(1+R)/(1−R),where R is the reflectivity of the coatings and τ_(d) is the fibertransmission delay. The free spectral range is:${{FSR}_{r} = \frac{c}{2n_{eff}L_{r}}},$where c is the speed of light in vacuum, n_(eff) is the effective indexof the resonator, and L_(r) is the length of the resonator.

For a reasonable reflectivity of R=0.99, τ_(eff)=199τ_(d). Hence, for agiven fiber length, the effective delay time is increased by 199 times,or for a given required energy storage time τ_(eff) the required thefiber length is reduced by 199 times. With proper reflective coatings, afiber Fabry-Perot resonators with a 20-meter long fiber can be used tohave an effective delay time equivalent to a delay by a 3-km long fiber.

Another configuration for the optical resonator 121 is a fiber ringresonator shown in FIG. 7. Such a ring resonator can be fabricated bycoupling two fiber directional couplers 710 and 712 to a fiber ring 714.Light coupled into the fiber ring 714 circulates in the ring 714 manytimes before exiting. The resulting effective energy storage timedepends on the coupling ratios and excess losses of the couplers 710 and712. The free spectral range for the ring resonator is given by:${{FSR}_{r} = \frac{c}{n_{eff}L_{r}}},$where L_(r) is the circumference of the ring 714. FIGS. 7A and 7B showtwo exemplary single-loop oscillators based on the fiber ring resonator.

FIG. 8A shows a micro whispering-gallery-mode resonator 800 with evensmaller size and weight than the above resonators. This resonator 800includes a transparent micro sphere, a ring, or a disk 801 as the cavityand two optical couplers 802 and 804. Quality-factor of such resonatorsis limited by optical attenuation in the material and scattering onsurface inhomogeneities, and can be as high as 10⁴−10⁵ in microrings anddisks, and up to 10¹⁰ in microspheres. See, e.g., Suzuki et al., IEEEPhoton. Technol. Lett., Vol. 4, pp.1256-1258 (1992); Little et al., J.Lightwave Technol., Vol.15, pp.998-1005 (1997); Braginsky et al.,Phys.Lett. A, v.137, pp.393-397 (1989); and Gorodetsky et al., Opt.Lett., vol.21, pp.453-455 (1996). The material for the cavity 801 may bea variety of dielectric materials, including fused silica which is a lowloss material for optical fibers. Each coupler may be a prism or inother forms.

Light is coupled into and out of the micro resonator 800 inwhispering-gallery modes through the evanescent fields at the surface ofthe sphere 801 that decays exponentially outside the sphere 801. Oncecoupled into the sphere 801, the light undergoes total internalreflections at the surface of the sphere 801 in a similar fashion aslight propagating in an optical fiber. The effective optical path lengthis increased by such circulation, just like in a fiber ring resonator.

It is predicted that the effective path length of a micro resonator of afew hundreds of microns in diameter operating at 1550 nm can be as longas 10 km, limited by the intrinsic attenuation of the material. It hasalso been shown that high-Q microspheres can effectively replacefiber-optic delays in the OEO with a length up to 25 km, whichcorresponds to a Q factor of 19 million at 30 GHz. Such a high Qresonator can be used to achieve a phase noise of less than −60 dB at 1Hz away from a 30 GHz carrier in an OEO to meet the requirement of deepspace Ka band communication.

FIG. 8B shows an alternative microsphere resonator 810 using two fibers810 and 820 as the couplers. The end surfaces of both fiber couplers 810and 820 are cut at a desired angle and are polished to formmicro-prisms. The two fiber couplers 810 and 820 may be implemented byusing two waveguides formed in a substrate which can be used tointegrate the OEO on a single chip.

The whispering-gallery-mode resonators shown in FIGS. 8A and 8B can beused to form a special compact OEO integrated to a substrate, inaddition to their functions as an optical resonator in an OEO. Inparticular, when properly configured, the opto-electronic feedback loopcan be simplified, e.g., by eliminating the electrical amplifier 130shown in the OEO 100 of FIG. 1.

FIG. 9 illustrates an embodiment of an integrated OEO 900 with all itscomponents fabricated on a semiconductor substrate 901. The integratedOEO 900 includes a semiconductor laser 910, a semiconductorelectro-absorption modulator 920, a first waveguide 930, a microresonator 940 in whispering gallery modes, a second waveguide 950, and aphotodetector 960. An electrical link 970, e.g., a conductive path, isalso formed on the substrate 901 to electrically couple the detector 960to the modulator 920. The micro resonator 940 can be a microsphere, amicro disk, or a ring and operates in the whispering-gallery modes. Itis used as a high-Q energy storage element to achieve low phase noiseand micro size. A RF filter may be disposed in the link 970 to ensure asingle-mode oscillation. In absence of such a filter, a frequencyfiltering effect can be achieved by narrow band impedance matchingbetween the modulator 920 and the detector 960.

Both waveguides 930 and 950 have coupling regions 932 and 952,respectively, to provide proper optical coupling at two differentlocations in the micro resonator 940. The first waveguide 930 has oneend coupled to the modulator 920 to receive the modulated optical outputand another end to provide an optical output of the OEO 900. The secondwaveguide 950 couples the optical energy from the micro resonator 940and delivers the energy to the detector 960.

The complete closed opto-electronic loop is formed by the modulator 920,the first waveguide 930, the micro resonator 940, the second waveguide950, the detector 960, and the electrical link 970. The phase delay inthe closed loop is set so that the feedback signal from the detector 960to the modulator 920 is positive. In addition, the total open loop gainexceeds the total losses to sustain an opto-electronic oscillation. Thepreviously described mode matching conditions are also required.

In general, an electrical signal amplifier can be connected between thedetector 960 and the modulator 920. Photodetectors and modulators areusually terminated with a 50-ohm impedance to match that of thetransmission line or other microwave components, although the intrinsicimpedance of the detector 960 and modulator 920 are high, e.g., around afew kilo-ohms. Consequently, the generated photovoltage by thephotodetector 960, which equals to its photocurrent multiplied by 50ohm, is too low to efficiently drive the modulator 920. Thus, it isbecomes necessary to use a signal amplifier in the link 970 in order toproperly drive the modulator 920.

However, such a high-power element can be undesirable in a highlyintegrated on-chip design such as the OEO 900. For example, the highpower of the amplifier may cause problems due to its high thermaldissipation. Also, the amplifier can introduce noise or distortion, andmay even interfere operations of other electronic components on thechip.

One distinctive feature of the OEO 900 is to eliminate such a signalamplifier in the link 970 by matching the impedance between theelectro-absorption modulator 920 and the photodetector 960 at a highimpedance value. The desired matched impedance is a value so that thephotovoltage transmitted to the modulator 920, without amplification, issufficiently high to properly drive the modulator 920. In certainsystems, for example, this matched impedance is at about 1 kilo ohm orseveral kilo ohms. The electrical link 970 is used, without a signalamplifier, to directly connect the photodetector 960 and the modulator920 to preserve their high impedance. Such a direct electrical link 970also ensures the maximum energy transfer between the two devices 920 and960. For example, a pair of a detector and a modulator that are matchedat 1000 ohm has a voltage gain of 20 times that of the same pair thatare matched at 50 ohm.

The previously-described frequency control circuit 310 shown in FIGS.3A, 3B, and 3C may be similarly implemented in the OEO 900 of FIG. 9 tomaintain the mode matching condition (1). To adjust the cavity length ofthe micro resonator 940, the output signal of the circuit 310 may beused to cause a mechanical squeeze on the resonator 940, e.g., through apiezo-electric transducer.

The above implementations of an optical resonator and a frequencycontrol circuit in a single-loop OEO can be used in OEOs in otherconfigurations to reduce phase noise in opto-electronic oscillationsunder proper mode matching conditions. These other OEOs include, but arenot limited to, multi-loop OEOs, Brillouin OEOs, and coupled OEOs. Thefollowing are some exemplary implementations in these different EOs.

Multi-loop OEOs use at least one fiber loop in one opto-electronicfeedback loop of at least two feedback loops as an energy storageelement. Such devices are disclosed in the U.S. Pat. No. 5,777,778 toYao, which is incorporated herein by reference in its entirety. Thedifferent feedback loops have different delays. The opto-electronicfeedback loop with the longest delay is used to achieve low phase noiseand narrow spectral linewidth. This loop is also used to provide finefrequency tuning since its mode spacing is smaller than any of the otherfeedback loops. On the other hand, the feedback loop with the shortestdelay and the widest mode spacing, either opto-electronic or purelyelectronic, is used to provide a coarse frequency tuning to achieve awide continuous tuning range. The total open loop gain of the multipleloops must exceed the total losses to sustain an opto-electronicoscillation but each loop may have an open loop gain less then the lossin that loop.

One embodiment of the present invention is to place the opticalresonator 121 of the OEO 100 in FIG. 1 into an optical section of anopto-electronic loop in a multi-loop OEO as suggested by the U.S. Pat.No. 5,777,778 to further reduce the phase noise and the spectrallinewidth of the opto-electronic oscillation. This optical resonator 121may be placed in the opto-electronic loop with the longest delay toreduce the amount of the fiber, the physical size, and cost of the OEO.In addition to satisfy the previously described three mode matchingconditions, an oscillating mode must be in resonance with one mode ineach feedback loop, i.e., one mode of each loop must overlap one modefrom each and every of other loops. The frequency control circuit 310shown in FIGS. 3A, 3B, and 3C may be similarly implemented to maintainthe mode matching condition (1)

FIGS. 10A and 10B show two examples of dual-loop OEOs having an opticalresonator 121. The OEO in FIG. 10A has two opto-electronic loops 1001and 1002 that respectively drive two electrical ports on the EOmodulator 110 to control the optical modulation. The optical resonator121 is placed in the longer loop 1002. A RF signal splitter, such as abiased Tee, can be coupled between the detector 127 and the amplifier130 to transmit high frequency signal component to the modulator 110 andto direct low frequency component to the frequency control circuit 310which responds to this input to control the laser 101. Otherconfigurations of implementing the circuit 310 are also possible in viewthe previous description.

FIG. 10B shows a dual-loop OEO with one electrical loop and oneopto-electronic loop. An electrical signal combiner 1010 is used tocombine the electrical signals of the two loops to produce a sum signalto drive a single port in the EO modulator 110. In general, a signalcombiner can used in this manner to combine the electrical signals fromtwo or more loops together. Similar to the system in FIG. 10A, thefrequency control circuit 310 here receives its input from a lowfrequency output of a biased Tee coupled between the detector 127 andthe coupler 1010 to control the laser 101.

Brillouin Opto-electronic oscillators use at least one activeopto-electronic feedback loop that generates an electrical modulationsignal based on the stimulated Brillouin scattering in a Brillouinoptical medium in the loop. See, e.g., U.S. Pat. No. 5,917,179 to Yao,which is incorporated by reference in its entirety. An optical pumplaser beam is injected into the Brillouin optical medium to produce anacoustic grating moving in the direction of the pump laser beam due tothe electrorestrictive effect. The grating interacts with the pump laserbeam to produce a backscattered Brillouin scattering signal at afrequency ν_(B) less than that of the pump laser beam ν_(P) by a Dopplershift ν_(D), i.e., ν_(B)=ν_(P)−ν_(D). The Brillouin scattering signal isconverted into an electrical modulation signal by a photodetector in theopto-electronic feedback loop.

FIG. 11 shows a Brillouin OEO implementing an optical resonator 121 anda frequency control circuit 310 according to one embodiment of theinvention. The EO modulator 1101 uses the electrical modulation signalof the feedback loop 1104 to modulate an optical carrier produce by alaser 101 to generate a modulated optical carrier signal which ismodulated at an oscillation frequencyf_(OSC)=|ν_(B)−ν_(S)|=|ν_(P)−ν_(S)−ν_(D)|. The Brillouin medium is asegment of optical fiber 1103 in the loop 1104. The pump laser 1112 iscoupled into the fiber 1103 by a coupler 1110 in an opposite directionto the direction of the modulated optical carrier coupled into the loop1104. The Brillouin scattering signal is in the direction of the opticalcarrier signal. The photodetector 127 receives the Brillouin scatteringsignal and the optical carrier signal to produce the electricalmodulation signal. The frequency control circuit 310 is coupled toreceive an electrical signal converted from a portion of the opticaltransmission of the resonator 121.

FIG. 12 shows the mode matching conditions of the Brillouin OEO 1100. Inaddition to the mode matching conditions stated in FIGS. 2B-2D, thefollowing mode matching condition must be satisfied:f _(OSC)=|ν_(B)−ν_(S)|=|ν_(P)ν_(S)−ν_(D) |=J·FSR _(r)where J is an integer. Hence, the oscillation frequency f_(OSC) istunable by adjusting either the frequency ν_(P) of the pump laser 1112or the frequency ν_(S) of the signal laser 101, or both.

The Brillouin OEO 1100 uses two separate lasers 101 and 1112. Thisrequires that the frequencies of the two lasers be relatively stablewith respect to each other. FIG. 13 shows a Brillouin OEO 1300 that usesa single laser to produce both the pump laser and the signal laser. Anoptical circulator 1303 is used to couple a portion of the output of thelaser into the loop 1310 as the pump beam. The additional mode matchingdue to the Brillouin effect is modified to f_(OSC)=ν_(D)=J·FSR_(r).Hence, the oscillation frequency f_(OSC) is determined by Doppler shiftand is in general not tunable.

One or more auxiliary feedback loops may be implemented in addition tothe Brillouin opto-electronic feedback loop to form a multi-loopBrillouin OEO. An auxiliary feedback loop may be of any type, includingan electrical feedback loop, an optical loop, a non-Brillouinopto-electronic loop, or another Brillouin opto-electronic loop. Eachloop may have an open loop gain smaller than unity and is still capableof sustaining an oscillation as long as the total open loop gain of allloops is greater than unity. FIG. 14 shows an exemplary dual-loopBrillouin OEO 1400 having a Brillouin opto-electronic loop 1420 and anon-Brillouin opto-electronic loop 1410. Similar to the system in FIG.10A, a bias Tee is coupled at the output of the detector 127 to send alow frequency component to the frequency control circuit 310.

Another type of OEOs is coupled opto-electronic oscillators (“COEOs”).See, e.g., U.S. Pat. No. 5,929,430 to Yao and Maleki, which isincorporated by reference in its entirety. Such a COEO directly couplesa laser oscillation in an optical feedback loop to an electricaloscillation in an opto-electronic feedback loop. The laser oscillationand the electrical oscillation are correlated with each other so thatboth the modes and stability of one oscillation are coupled with thoseof the other oscillation. The optical feedback loop includes a gainmedium to produce a loop gain greater than unity to effectuate the laseroscillation. This optical loop may be a Fabry-Perot resonator, a ringresonator, other resonator configurations. The open loop gain in theopto-electronic loop also exceeds the loss to sustain the electricaloscillation. The coupling between two feedback loops is achieved bycontrolling the loop gain of the optical loop by an electrical signalgenerated by the opto-electronic feedback loop. COEOs can achieve asingle-mode RF oscillation without a RF bandpass filter or anyadditional opto-electronic feedback loops. A multi-mode laser can beused.

In comparison with other exemplary OEOs described above, an opticalresonator can be placed in either an opto-electronic feedback loop or inthe optical feedback loop of a COEO. The former configuration requireslocking the frequencies of the laser modes to the transmission peaks ofthe resonator by using a frequency control circuit. Alternatively, twooptical resonators may be respectively placed in the optical loop andthe opto-electronic loop at the same time.

FIG. 15A shows one embodiment of a COEO 1500 having an optical feedbackloop 1510 and an opto-electronic loop 1520. An optical resonator 121 isplaced in the optical loop 1510. The optical loop 1510 is shown to be aring laser that includes an optical amplifier 1512 and an EO modulator1514. An optical isolator 1516 may be used to ensure the optical wave inthe loop 1510 is unidirectional. The ring may be formed by optical fiber1511 or other optical waveguides. The optical amplifier 1512 and the EOmodulator 1514 in combination effectuate a laser gain medium whose gaincan be controlled and modulated by an electrical control signal from theopto-electronic loop 1520. A semiconductor optical amplifier, forexample, can be used to function as the combination of the amplifier1512 and the modulator 1514.

One of the advantages of placing the optical resonator 121 in theoptical loop 1510 is that the optical modes inside the optical loop 1510are controlled by the modes of the resonator 121, i.e., only the modesof the loop 1510 that overlap with the modes of the resonator 121 canhave enough gain to oscillate. Therefore, the optical frequencies of thelaser are automatically aligned with the transmission peaks of theresonator 121. This configuration eliminates the need to lock theoptical loop 1510 and the resonator 121 relative to each other in otherOEOs with an optical resonator. In addition, the resonator 121 insidethe optical loop 1510 determines the quality of both the optical signalgenerated in the optical loop 1510 and the RF signal generated in theopto-electronic loop 1520. This is because both signals share the verysame energy storage element, the resonator 121. For a fiber amplifierbased COEO, high-Q optical resonators constructed with optical fiber arepreferred, including the fiber Fabry-Perot resonator in FIG. 6 and thefiber ring resonator in FIG. 7.

FIG. 15B illustrate the spectral relationships of the resonatortransmission, the laser modes, the opto-electronic oscillation modes,and the RF filter's center frequency and bandwidth. Because the cavitylength of the optical loop 1510 includes the contribution of theresonator length, its mode spacing is generally smaller than the FSR ofthe resonator 121. Consequently, only those of the laser modes thatcoincide with the transmission peaks of the resonator 121 can oscillate.

The opto-electronic feedback loop 1520 is generally much longer than thecavity length of the laser 1510 and hence a corresponding mode spacingis smaller than the mode spacing of the ring laser 1510. The centerfrequency of the RF bandpass filter 160 is chosen to be equal to a RFbeat frequency of different modes of the ring laser 1510. The bandwidthof the filter 160 is chosen to be narrower than the spacing of the modebeat frequencies (equivalent to FSR of the resonator 121). Within thepass band, many OEO modes compete to oscillate. However, the winner isthe mode with a frequency closest to a beat frequency of the laser'slongitudinal modes, since only this OEO mode can get energy from thelaser 1510. This mode is fed back to modulate the gain of the ring laser1510, and it effectively mode-locks the ring laser. The mode-lockingmakes the mode spacing of the laser equal to the frequency of theoscillating OEO mode, which is a multiple of the FSR of the resonator.

Because all the oscillating modes in the mode-locked laser 1510 areforced to be in phase, all the mode beat signals between any twoneighboring laser modes thus add up in phase and generate a strongsignal at the frequency of the oscillating OEO mode. This enhanced modebeat signal in turn provides more gain to the oscillating OEO mode andreinforces its oscillation.

Therefore, the conditions for oscillation are:

-   -   (1) the total length of the optical loop 1510 (laser cavity) is        the multiple of the cavity length of the resonator 121 (or the        free spectral range of the resonator 121 is multiple of laser's        mode spacing);    -   (2) the length of the opto-electronic loop 1520 is the multiple        of the resonator's cavity length (or the free spectral range of        the resonator is the multiple of opto-electronic loop's mode        spacing);    -   (3) the center frequency of the RF filter 160 is the multiple of        the free spectral range of the resonator 121; and    -   (4) the bandwidth of the RF filter 160 is less than the FSR of        the resonator 121.

FIG. 15C shows a COEO having two optical resonators based on the systemin FIG. 15A. A frequency control circuit 310 is used to lock the modesof the optical resonator 1530 in the opto-electronic loop 1520 relativeto the modes of the optical loop 1510. In this example, the control canbe achieved by controlling the total cavity length of the optical loop1510. In addition to the above mode matching conditions, the modespacing of the optical resonator 1530 must be a multiple of the lasermodes and the modes of the resonator 1530 and the modes of the resonator121 must have some overlapped modes.

FIG. 16 shows an embodiment of a COEO-on-chip 1600 based on a microcavity in whispering gallery modes. This device is formed on asemiconductor substrate 1601 and includes two waveguides 1610 and 1620that are coupled to a high Q micro cavity 1602 such as a microsphere asshown. The waveguides 1610 and 1620 have angled ends 1616 and 1626,respectively, to couple to the micro cavity 1602 by evanescent coupling.The other end of the waveguide 1610 includes an electrical insulatorlayer 1611, an electro-absorption modulator section 1612, and a highreflector 1614. This high reflector 1614 operates to induce pulsecolliding in the modulator 1612 and thus enhance the mode-lockingcapability. The other end of the waveguide 1620 is a polished surface1624 and is spaced from a photodetector 1622 by a gap 1621. The surface1624 acts as a partial mirror to reflect a portion of light back intothe waveguide 1620 and to transmit the remaining portion to thephotodetector 1622 to produce an optical output and an electricalsignal. An electrical link 1630 is coupled between the modulator 1612and photodetector 1622 to produce an electrical output and to feed thesignal and to feed the electrical signal to control the modulator 1612.

Hence, two coupled feedback loops are formed in the device 1600. Anoptical loop is in the form of a Fabry-Perot resonator configuration,which is formed between the high reflector 1614 and the surface 1624 ofthe waveguide 1620 through the modulator 1612, the waveguide 1610, themicro cavity 1602, and the waveguide 1620. The gap 1621, the detector1622, and the electrical link 1630 forms another opto electronic loopthat is coupled to the optical loop. Hence, the configuration of theCOEO 1600 is similar to the configuration uses in the COEO 1500 in FIG.15A. Therefore, the same mode matching conditions apply.

The waveguides 1610 and 1620 are active and doped to also function asthe gain medium so that the optical loop operates as a laser whenactivated by a driving current. This current can be injected from properelectrical contacts coupled to an electrical source. The gain of thelaser is modulated electrically by the modulator 1612 in response to theelectrical signal from the photodetector 1622.

The photodetector 1622 can be structurally identical to theelectro-absorption modulator 1612 but is specially biased to operate asa photodetector. Hence, the photodetector 1622 and the modulator 1612have a similar impedance, e.g., on the order of a few kilo ohms, andthus are essentially impedance matched. Taking typical values of 2 voltsmodulator switching voltage, 1 kilo ohm for the impedance of themodulator 1612 and photodetector 1622, the optical power required forthe sustained RF oscillation is estimated at about 1.28 mW when thedetector responsivity is 0.5 A/W. Such an optical power is easilyattainable in semiconductor lasers. Therefore, under the impedancematching condition, a RF amplifier can be eliminated in the electricallink 1630 as in the integrated OEO 900 in FIG. 9.

The two waveguides 1610 and 1620 may be positioned adjacent and parallelto each other on the substrate 1601 so that the photodetector 1622 andthe modulator 1612 are close to each other. This arrangement facilitateswire bonding or other connection means between the photodetector 1622and the modulator 1612.

Although only a few embodiments are described, various modifications andenhancements may be made without departing from the following claims.

1. A opto-electronic device, comprising: a substrate formed of asemiconductor; a semiconductor laser formed on said substrate to producea laser beam; a semiconductor optical modulator formed on said substrateto receive and modulate said laser beam from said semiconductor laser inresponse to an electrical modulation signal; a first waveguide formed onsaid substrate and coupled to receive a modulated optical signal fromsaid optical modulator; an optical resonator formed on said substrateand coupled to said first waveguide to receive a portion of saidmodulated optical signal; a second waveguide formed on said substrateand coupled to receive a portion of said modulated optical signal insaid optical resonator; a semiconductor photodetector formed on saidsubstrate to receive and convert an optical output from said secondwaveguide into an electrical signal; and an electrical link formed onsaid substrate and coupled between said photodetector and said opticalmodulator to produce said electrical modulation signal from saidelectrical signal.
 2. A device as in claim 1, wherein said opticalresonator is a micro resonator in whispering gallery modes.
 3. A deviceas in claim 2, wherein said micro resonator is a dielectric sphere.
 4. Adevice as in claim 2, wherein said first and second waveguides includecoupling sections adjacent to said micro resonator to couple opticalenergy to and from said micro resonator via evanescent coupling.
 5. Adevice as in claim 1, wherein said photodetector and said opticalmodulator have a substantially matched impedance.
 6. A device as inclaim 1, wherein said electrical link does not have a signal amplifierbetween said photodetector and said optical modulator.
 7. A device as inclaim 1, wherein said optical modulator is an electro absorptionmodulator.
 8. A device as in claim 7, wherein said photodetector is adevice which is structurally identical to said electro absorptionmodulator but is biased to operate as a photodetector.
 9. A device as inclaim 1, wherein said optical resonator has a free spectral range thatis greater by a factor of an integer than a mode spacing of modes in anopto-electronic loop formed by said optical modulator, said first andsecond waveguides, said photodetector, and said electrical link.
 10. Aopto-electronic device, comprising: a substrate formed of asemiconductor; a semiconductor optical modulator formed on saidsubstrate to modulate an optical beam in response to an electricalmodulation signal, said optical modulator having a first side and anopposing second side that define an optical path; an optical reflectorformed on said first side of said optical modulator; a first waveguideformed on said substrate having a first end that is to receive amodulated optical signal from said optical modulator and is insulatedfrom said optical modulator, and a second end that has an angled facet;an optical resonator operating in whispering gallery modes and formed onsaid substrate and coupled to said angled facet of said first waveguidevia evanescent coupling; a second waveguide formed on said substrate andhaving a first end with an angled facet which is coupled to said opticalresonator via evanescent coupling, and a second end; a semiconductorphotodetector formed on said substrate and spaced from said second endof said second waveguide to receive and convert an optical output fromsaid second waveguide into an electrical signal; and an electrical linkformed on said substrate and coupled between said photodetector and saidoptical modulator to produce said electrical modulation signal from saidelectrical signal, wherein said first and second waveguides are doped toproduce an optical gain to produce a laser oscillation in a laser cavityformed between said optical reflector and said second end of said secondwaveguide.
 11. A device as in claim 10, wherein said photodetector andsaid optical modulator have a substantially matched impedance.
 12. Adevice as in claim 10, wherein said electrical link does not have asignal amplifier between said photodetector and said optical modulator.13. A device as in claim 10, wherein said optical modulator is anelectro absorption modulator.
 14. A device as in claim 13, wherein saidphotodetector is a device which is structurally identical to saidelectro absorption modulator but is biased to operate as aphotodetector.
 15. A device as in claim 10, wherein said opticalresonator has a free spectral range that is greater by a factor of aninteger thin a mode spacing of modes in an opto-electronic loop formedby said optical modulator, said first and second waveguides, saidphotodetector, and said electrical link.
 16. A device as claimed inclaim 10, wherein said optical resonator is a sphere formed of adielectric material.